1. Field of the Invention
The present invention generally relates to improvements in railroad telemetry and control systems and, more particularly, to improvements in End of Train (EOT) units mounted on the last car of a train and Head of Train (HOT) units mounted in the cab of a locomotive. An improved protocol allows EOT units having different code formats to be used with the HOT unit. The EOT unit incorporates a self-calibration feature, and the HOT unit, cooperating with the EOT unit, provides an output to the train crew indicating the approximate location of a fault in the brake system causing an Undesired Emergency (UDE) brake operation.
2. Description of the Prior Art
End of Train (EOT) signaling and monitoring equipment is now widely used, in place of cabooses, to meet operating and safety requirements of railroads. The information monitored by the EOT unit typically includes the air pressure of the brake line, battery condition, warning light operation, and train movement. This information is transmitted to the crew in the locomotive by a battery powered telemetry transmitter.
The original EOT telemetry systems were one-way systems; that is, data was periodically transmitted from the EOT unit to the Head of Train (HOT) unit in the locomotive where the information was displayed. More recently, two-way systems have been introduced wherein transmissions are made by the HOT unit to the EOT unit. In one specific application, the EOT unit controls an air valve in the brake line which can be controlled by a transmission from the HOT unit. In a one-way system, emergency application of the brakes starts at the locomotive and progresses along the brake pipe to the end of the train. This process can take significant time in a long train, and if there is a restriction in the brake pipe, the brakes beyond the restriction may not be actuated. With a two-way system, emergency braking can be initiated at the end of the train independently of the initiation of emergency braking at the head of the train, and the process of brake application can be considerably shortened. As will be appreciated by those skilled in the art, in order for a HOT unit to communicate emergency commands to an associated EOT unit, it is desirable for the HOT unit to be "armed"; that is, authorized by railroad personnel. This is desirable to prevent one HOT unit from erroneously or maliciously actuating the emergency brakes in another train. To this end the HOT unit includes a nonvolatile memory in which a unique code identifying an EOT unit can be stored. The HOT unit also has a row of thumb wheel switches.
A logistical problem arises for various railroads which use EOT and HOT units made by different manufacturers. Although the Association of American Railroads (AAR) Communication Manual establishes standards for the communication protocol between EOT units and HOT units, those standards allow for the inclusion of discretionary information. This discretionary information is different for various manufacturers resulting in the possibility of the transmission from an EOT unit from one manufacturer having some degree of incompatibility with the HOT unit installed in the locomotive. In addition, there are currently in the field many EOT units which are of the earlier one-way transmission variety, and a number of those units use a protocol which is completely different from the AAR specification.
U.S. Pat. No. 4,885,689 to Kane et al. discloses a telemetry receiver which is capable of automatically recognizing certain incompatible code formats and correctly decoding received data from one-way EOT units. This telemetry receiver has been incorporated into HOT units and has provided a measure of compatibility between the EOT units of different manufactures and the HOT unit installed in a locomotive. However, further compatibility problems have arisen since the Kane et al. invention as a result of the introduction of two-way transmission systems.
Currently, there are several protocols in active use on North American railroads. These include two variants of the AAR two-way protocol, specifically one used in Canada and one used by the assignee of this application in the United States, two AAR one-way protocols differing in the discretionary bits employed, the one-way protocol implemented by the assignee of this application and described in the above-referenced Kane et al. patent, and a two-way protocol developed by the assignee of this application. This proliferation of protocols has exacerbated the compatibility problem.
The use of EOT and HOT units has presented the possibility of solving a problem of Undesired Emergency (UDE) brake operations by assisting in the location of the fault causing the UDE. The AAR has released a study of UDEs as has the Canadian Air Brake Club, which references the work by the AAR. According to the AAR study, UDEs are normally sporadic and unpredictable, and finding the control valve which initiated the UDE is an almost impossible task. The Canadian Air Brake Club has proposed a method of determining UDE location for trains equipped with EOT units which is based on the propagation times for a pressure loss wave to reach the EOT unit and the HOT unit. Using the proposed method, an informed inspector/supervisor riding an EOT unit equipped train subject to UDEs has a simple investigative tool requiring only a stop watch, constant attention and presence of mind, according to the Canadian Air Brake Club report. The Canadian Air Brake Club also suggest that if locomotive crews developed the automatic habit of counting the seconds difference between front and rear emergency indications, the source of the UDE could also be roughly located prior to walking the train to remedy the situation. For those locomotives equipped with event recorders for after-the-fact investigation, the Canadian Air Brake Club proposes developing a "suspect car" database in order to identify and weed out marginally stable valves. This database would be developed by downloading .data from event recorders which record UDEs and identifying repeat cars in the database as "suspect cars".
The increased reliance on EOT units in train monitoring and control means that these devices have become an indispensable safety item in the operation of trains. It is therefore important that they operate both reliably and accurately. Accurate operation requires that the EOT units be properly calibrated, and this has been done in the past by specially trained personnel. What is needed is an automatic calibration feature which would not require specially trained personnel.